The osteopenia that accompanies the menopause continues to represent a major public health problem. Left unchecked, the cumulative loss of bone can potentially compromise the skeleton's structural integrity, resulting in painful and debilitating fractures of the wrist, spine and femur. Efforts to reduce the risk and incidence of fractures have focused on the development of therapies that conserve skeletal mass by inhibiting bone resorption. Among various treatment modalities, estrogen replacement therapy remains the preferred means to prevent the development of post menopausal osteoporosis (Lindsey R, Hart D M, MacClean A 1978, "The role of estrogen/progestogen in the management of the menopause", Cooke I D, ed, Proceedings of University of Sheffield symposium on the role of estrogen and progestogen in the management of the menopause, Lancaster, UK: MTP Press Ltd. pp. 9-25; Marshall D H, Horsmann A, Nordin B E C 1977, "The prevention and management of post-menopausal osteoporosis.", Acta Obstet Gynecol Scand (Suppl) 65: 49-56; Recker R R, Saville P D, Heaney R P 1977, "Effect of estrogen and calcium carbonate on bone loss in post-menopausal women", Ann Intern Med. 87: 649-655; Nachtigall L E, Nachtigall R H, Nachtigall R D, Beckman E M 1979, "Estrogen replacement therapy", Obstet Gynecol. 53: 277-281) and it is now accepted that estrogens significantly decrease fracture incidence and risk (Krieger N, Kelsey J L, Holford T R, O'Connor T 1982, "An epidemiological study of hip fracture in postmenopausal women", Am J Epidemiol. 116: 141-148; Hutchinson T A, Polansky S M, Feinstein A R 1979, "Post-menopausal estrogens protect against fractures of hip and distal radius: A case-control study", Lancet 2: 705-709; Paginini-Hill A, Ross R K, Gerkins V R, Henderson B E, Arthur M, Mack T M 1981, "Menopausal oestrogen therapy and hip fractures", Ann Intern Med. 95: 28-31; Weiss N S, Ure C L, Ballard J H, Williams A R, Daling J R 1980, "Decreased risk of fractures on the hip and lower forearm with post-menopausal use of estrogen", N Eng J Med. 303: 1195-1198).
While the beneficial actions of estrogen replacement therapy on the skeleton are clearly significant, there is also considerable evidence for a positive effect of estrogen on the cardiovascular system. Previous studies have attributed these actions to estrogen's effects on serum lipids, but recent data has now shown that in addition to the effects on the lipid profile, estrogen can also directly influence vessel wall compliance, reduce peripheral resistance and prevent atherosclerosis (Lobo R A 1990, "Cardiovascular implication of estrogen replacement therapy", Obstetrics and Gynaecology, 75: 18S-24S; Mendelson M E, Karas R H 1994, "Estrogen and the blood vessel wall", Current Opinion in Cardiology, 1994(9): 619-626). Based on available epidemiological data, the overall impact of these physiological and pharmacological actions of estrogen is an age independent reduction in cardiovascular mortality and morbidity in women (Kannel W H, Hjortland M, McNamara P M 1976 "Menopause and risk of cardiovascular disease: The Framingham Study", Ann Int Med, 85: 447-552). Furthermore, a more recent analysis has concluded that post-menopausal estrogen replacement therapy reduces the risk of cardiovascular disease by approximately 50 percent (Stampfer M J, Colditz G A 1991, "Estrogen replacement therapy and coronary heart disease: a quantitative assessment of the epidemiological evidence", Preventive Medicine, 20: 47-63.).
In addition to the positive effects of estrogen on bone and cardiovascular system, there are now data which indicate that the central nervous system can benefit from estrogen replacement therapy. Short term studies in human subjects have shown that increased levels of estrogen are associated with higher memory scores in post menopausal women (Kampen D L, Sherwin B B 1994, "Estrogen use and verbal memory in healthy postmenopausal women", Obstetrics and Gynecology, 83(6): 979-983). Furthermore, the administration of exogenous estrogen to surgically post menopausal women specifically enhances short-term memory. Moreover, the effects of estrogen on cognition do not appear confined to short-term effects as epidemiological findings indicate that estrogen treatment significantly decreases the risk of senile dementia-Alzheimer's type in women (Paganini-Hill A, Henderson V W, 1994, "Estrogen deficiency and risk of Alzheimer's disease in women", Am J Epidemiol, 140: 256-261; Ohkura T, Isse K, Akazawa K, Hamamoto M, Yoshimasa Y, Hagino N, 1995, "Long-term estrogen replacement therapy in female patients with dementia of the Alzheimer Type: 7 case reports", Dementia, 6: 99-107). While the mechanism whereby estrogens enhance cognitive function is unknown, it is possible to speculate that the direct effects of estrogen on cerebral blood flow (Goldman H, Skelley E b, Sandman C A, Kastin A J, Murphy S, 1976, "Hormones and regional brain blood flow", Pharmacol Biochem Rev. 5(suppl 1): 165-169; Ohkura T, Teshima Y, Isse K, Matsuda H, Inoue T, Sakai Y, Iwasaki N, Yaoi Y, 1995, "Estrogen increases cerebral and cerebellar blood flows in postmenopausal women", Menopause: J North Am Menopause Soc. 2(1): 13-18) and neuronal cell activities (Singh M, Meyer E M, Simpkins J W, 1995, "The effect of ovariectomy and estradiol replacement on brain-derived neurotrophic factor messenger ribonucleic acid expression in cortical and hippocampal brain regions of female Sprague-Dawley rats", Endocrinology, 136: 2320-2324; McMillan P J, Singer C A, Dorsa D M, 1996, "The effects of ovariectomy and estrogen replacement on trka and choline acetyltransferase mRNA expression in the basal forebrain of the adult female Sprague-Dawley rat", J Neurosci., 16(5): 1860-1865) are potential effectors for these beneficial actions.
The therapeutic applications of naturally occurring estrogens and synthetic compositions demonstrating estrogenic activity alone or in combination are not limited to the chronic conditions described above. Indeed, the more traditional applications of estrogen therapies would include the following: relief of menopausal symptoms (i.e. flushing and urogenital atrophy); oral contraception; prevention of threatened or habitual abortion, relief of dysmenorrhea; relief of dysfunctional uterine bleeding; an aid in ovarian development; treatment of acne; diminution of excessive growth of body hair in women (hirsutism); treatment of prostatic carcinoma: and suppression of post-partum lactation [Goodman and Gilman, The Pharmacological Basis of Therapeutics (Seventh Edition) Macmillan Publishing Company, 1985, pages 1421-1423].
Even though the beneficial effects of estrogen replacement on a wide variety of organ systems and tissues appear indisputable, the dose and duration of estrogen therapy is also associated with an increased risk of endometrial hyperplasia and carcinoma. The use of concomitant cyclic progestins does reduce the risk of endometrial pathology, but this is achieved at the expense of the return of regular uterine bleeding, a result that is objectionable to many patients. In addition to estrogen's stimulatory effect on the endometrium, there remains considerable controversy regarding reports of an association between longterm estrogen replacement and an increased risk of breast cancer (Bergkvist L, Adami H O, Persson I, Hoover R, Schairer C, 1989, "The risk of breast cancer after estrogen and estrogen-progestin replacement", N Eng J Med, 321: 293-297; Colditz G A, Hankinson S E, Hunter D J, Willett W C, Manson J E, Stampfer M J, Hennekens C, Rosner B, Speizer F E, 1995, "The use of estrogens and progestins and the risk of breast cancer in postmenopausal women", N Eng J Med, 332(24): 1589-1593). Furthermore, there are other side effects of estrogen replacement which, while they may not be life threatening, contraindicate estrogen's use and reduce patient compliance.
From the foregoing discussion it would appear that the availability of therapies which could mimic the beneficial actions of estrogen on the bone, cardiovascular system, and central nervous system without the undesirable side effects on uterus and breast, would essentially provide a "safe estrogen" which could dramatically influence the number of patients that would be able to benefit from estrogen replacement therapy. Therefore, in recognition of estrogen's beneficial effects on a number of body systems and disease conditions, there is a continuing need for the development of potent estrogen agonists which can selectively target different body tissues and which display a high bioavailability.